1. Field of the Invention
This invention generally relates to early warning failure detection for optical amplifiers. More particularly, it relates to early warning failure detection for lasing semiconductor optical amplifiers, such as vertical lasing semiconductor optical amplifiers (VLSOAs), where the failure detection is based on detecting a shift in wavelength of a ballast laser signal generated by the amplifier.
2. Description of the Related Art
Optical amplifiers are a basic building block for many types of optical systems. For example, fiber optic communications systems transmit information over optical fibers. A typical communications system includes a transmitter, an optical fiber, and a receiver. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. In these systems, phenomena such as fiber losses, losses due to insertion of components in the transmission path, and splitting of the optical signal may attenuate the optical signal and degrade the corresponding signal-to-noise ratio as the optical signal propagates through the communications system. Optical amplifiers are used to compensate for these attenuations. As another example, receivers typically operate properly only within a relatively narrow range of optical signal power levels; optical amplifiers may be used to boost an optical signal to the proper power range for the receiver.
It is generally beneficial to monitor optical amplifiers to ensure that they are operating correctly. For example, one factor in the efficient utilization of an optical network is the ability to detect and correct failures within the network. Monitoring of optical amplifiers in the network can help locate a point of failure. Early warning before failures occur would also be beneficial, as this can be used to prevent failures; optical amplifiers which are identified as subject to failure in the near future can be replaced before they actually fail.
One method typically used to monitor an optical amplifier is based on tapping a small portion of the amplified optical signal leaving the optical amplifier. If the strength of the tapped portion falls within a specified range, this is an indication that the optical amplifier is operating correctly (or at least outputting a signal). In contrast, if the tapped portion is unusually weak or non-existent, this suggests that the optical amplifier may have failed. However, this approach reduces the optical signal""s strength since a portion of the optical signal is tapped for monitoring purposes. As optical networks expand and the number of amplifiers in a signal path increases, the cumulative effect of all of these tap losses can be significant. Another drawback to this approach is that it does not provide early warning of a future failure.
As a result, there is a need for a failure detection capability for optical amplifiers which does not introduce tap loss or other types of optical loss. There is also a need for a failure detection capability which provides early warning of failures. In the context of optical communications systems, early warning failure detection would allow re-routing of data traffic away from optical amplifiers before they fail. Additionally, early warnings provide more time for a network manager to replace a failed (or about to fail) optical amplifier.
In accordance with the present invention, early warning failure detection is provided for an optical amplifier. The optical amplifier is based on a lasing semiconductor optical amplifier, which generates a ballast laser signal in addition to the amplified optical signal. The ballast laser signal exhibits a wavelength shift before failure and this wavelength shift is used as the basis for an early warning of future failure of the amplifier.
In one embodiment, the optical amplifier with early warning failure detection includes a lasing semiconductor optical amplifier coupled to a wavelength-sensitive detector. The lasing semiconductor optical amplifier includes a semiconductor gain medium, an amplifying path which traverses the semiconductor gain medium, a laser cavity which includes the semiconductor gain medium, and a pump input to the semiconductor gain medium. When the semiconductor gain medium is pumped above threshold for the laser cavity, the laser cavity generates a laser output (i.e., the ballast laser signal) which acts as a ballast for the amplification process. Early warning failure detection is based on detecting a wavelength shift in the ballast laser signal. The wavelength-sensitive detector receives the ballast laser signal for this purpose.
In one implementation, the lasing semiconductor optical amplifier is a vertical lasing semiconductor optical amplifier (VLSOA). In another aspect of the invention, early warning of failure is indicated by a shift to a longer wavelength. In yet another aspect, the wavelength-sensitive filter is implemented as an optical filter followed by a detector. For example, the pre-shift version of the ballast laser signal may fall in the stop band of the optical filter and the post-shift version in the pass band, or vice versa. In yet another variation, a VLSOA, optical filter and detector are implemented as layers of different materials stacked on a common substrate, thus yielding an integrated device.
In another aspect of the invention, the efficiency with which an incoming pump current is converted into the ballast laser signal changes, typically decreasing, before failure and this change in conversion efficiency is used as the basis for an early warning of future failure of the amplifier. In one approach, the pump current is held constant and a decrease in the ballast laser signal then indicates a decrease in the conversion efficiency. In another approach, the pump current is adjusted so that the ballast laser signal is held constant. An increase in the amount of pump current required then indicates a decrease in the conversion efficiency.
The present invention is particularly advantageous because it provides early warning of future failure, thus allowing proactive steps to be taken before the actual failure of the optical amplifier. In addition, the early warning is provided without diverting a portion of the amplified signal. Thus, no tap loss is introduced.
Other aspects of the invention include methods based on the above and systems which include optical amplifiers with early warning failure detection capability. Examples of such systems include fiber optic communications systems, transmitters, receivers, and switching nodes.